Bipolar log converter

ABSTRACT

A bipolar log converter capable of receiving bidirectional current I and providing (a) a continuous potential signal having a polarity representative of the flow direction of the current, (b) the potential signal having a magnitude representative of the current, and (c) the potential signal changing polarity upon a change in the flow direction of the current. This converter is especially useful in a novel dynamic analyzer for evaluation of voltage-current related properties in an external system and, in a particular application, to a unique potentiostat employed in performing electrochemical analysis of corrosion phenomena in a test cell. The bipolar log converter employs a unique voltage network or bridge using series connected rectifier pair biased into a conducting state. The bidirectional current I flows in only one rectifier to produce a conversion logarithmically into a potential. This potential in the network produces at its output a potential signal + OR - V I. Temperature compensation for the network is also disclosed.

United States Patent Wilson Dec. 23, 1975 Related US. Application Data.

[62] Division of Ser. No. 436,250, Jan. 24, 1974, Pat. No.

[52] US. Cl. 307/229; 307/25 S; 328/145 [51] Int. Cl. G06G 7/12; G0667/24 [58] Field of Search..... 328/145; 307/229; 235/193, 235/ 197 [56]References Cited UNITED STATES PATENTS 3,329,836 7/1967 Pearlman 328/1453,448,289 6/1966 Harris 328/145 3,584,232 6/1971 Wallace 307/2293,681,618 8/1972 Blackmer 328/145 Primary ExaminerStanley D. Miller, Jr.Assistant Examiner-B. P. Davies Attorney, Agent, or FirmEmil J. Bednar57 ABSTRACT A bipolar log converter capable of receiving bidirectionalcurrent I and providing (a) a continuous potential signal having apolarity representative of the flow direction of the current, (b) thepotential signal having a magnitude representative of the current, and(c) the potential signal changing polarity upon a change in the flowdirection of the current. This converter is especially useful in a noveldynamic analyzer for evaluation of voltage-current related properties inan external system and, ina particular application, to a uniquepotentiostat employed in performing electrochemical analysis ofcorrosion phenomena in a test cell.

The bipolar log converter employs a unique voltage network or bridgeusing series connected rectifier pair biased into a conducting state.The bidirectional current I flows in only one rectifier to produce aconversion logarithmically into a potential. This potential in thenetwork produces at its output a potential signal :V 1. Temperaturecompensation for the network is also disclosed.

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U.S. Patent Dec. 23, 1975 US. Patent Dec. 23, 1975 Sheet 2 of4 3,928,774

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US. Patent Dec. 23, 1975 Sheet4 of4 3,928,774

BIPOLAR LOG CONVERTER This application is a division of application Ser.No. 436,250 filed Jan. 24, 1974, now US. Pat. No 3,855,101.

BACKGROUND OFTI-IE INVENTION 1. Field of the Invention This inventionrelates to measuring and testing electrochemical processes; and itrelates particularly to instruments for the practice of electrochemicaland electroanalytical techniques used in the study of corrosionprocesses in conductive media.

2. Description of the Prior Art In the field of electrochemical andelectroanalytical instruments and processes, many types of analyses canbe performed in external systems which have voltagecurrent relatedproperties. The study of corrosion phenomena is one such area. Otherareas include, but are not limited to, the following: phase-sensitiveAC, pulse, and DC polarography; anodic stripping analysis; cycling andpotential sweep voltammetry; pH and specific ion measurement; directpotentiometry; controlled potential and controlled current electrolysis;chronopotentiometry; chronoamperometry; pulse response studies;electrical double layer capacitance measurements; itensiostatic,potentiostatic, and potentiokinetic methods for corrosion studies; andperforming corrosion measurements as described in US. Pat. No.3,101,406.

Instruments for practicing these analyses may be denoted, in a mostgeneral sense, as potentiostats. Such instruments are arranged toproduce and maintain a given voltage within the external system havingvoltage-current related properties by regulation of the current flowingtherethrough. The potentiostatic instruments usually include a highimpedance voltmeter for determining the maintained potential, a currentsource capable of maintaining a current flow to insure a constant valuefor the induced potential, and various auxiliary equipment whichincludes the cells, electrodes, and'so forth, forming the externalsystem, and various types of readout devices (ammeters, voltmeters,recorders, scopes, etc.). The auxiliary equipment can also includetimers, recorders, and function generators capable of producing pulses,square waves, sawtooths and sine wave voltage sweeps. The readout meansinclude Oscilloscopes, various forms of wave analyzers, and impedancebridges.

The external system can be the classic types of electrochemical cellssuch as dropping mercury electrodes, hydrogen and glass referenceelectrodes, specific ion electrodes, metal electrodes and variouscombinations of such electrodes. These external systems all have acommon characteristic at their electrical terminals. The externalsystems exhibit voltage-current related properties at their terminals.In particular, a potential can be induced between a first pair ofterminals, and other terminals are employed for passing a currentthrough the cell which induces and maintains such potential. Themagnitude and direction of the current flow and its function with timehave a prescribed relationship to induced potential. These relatedproperties of voltage and current are definitive of the electrochemicaland electroanalytical composition of the cell.

The most common analysis of external systems having voltagecurrentrelated properties in aqueous media is voltammetry. In voltammetry, apair of electrodes are employed for sensing the induced potential in thesystem. Other electrodes are employed for passing current through theconductive media for inducing the potential between the first electrodepair. The induced potential may be maintained constant for a givenperiod of time, or it can be varied from a first, to a second, or evento a third, magnitude and varied at a constant rate with time, or withother functions with time such as exhibited by a sine wave or triangularwave.

Another electrochemical analysis of an external system found inmeasurements of corrosion phenomena is described in US. Pat. No.3,406,101. In this patent, there is described an external system formedby a corrosion cell containing an aqueous corrodant in which areimmersed three electrodes. Current is passed between two electrodes andinduces a potential relative to a third electrode (reference). Thecurrent flow required to induce a certain potential change between thereference and one other electrode (test) is employed to determine therate of corrosion which is occurring at the test electrode in the cell.Thus, the current flow in such a cell is the readout of the corrosionoccurring at the test electrode.

The known external systems having voltage-current related propertieshave a plurality of terminals and conventionally have at least fourterminals (e.g., two terminals to sense induced potential and twoterminals to maintain current flow). For example, four-electrodeconductivity cells are an external system having voltage-current relatedproperties in which the potentiostatic instruments find readyapplication.

Prior art instruments employed in the electrochemical andelectroanalytical field, particularly potentiostatic instruments, haveprovided useful results. However, these instruments left much to bedesired in easy and reliable operation. First, the induced potential inthe external system either had to be maintained at fixed levels forgiven lengths of time, and then changed with a square wave function toother levels in order to insure stable operation. Voltage sweeping hasbeen attained, for the most part, by motor-driven rheostats which sufferfrom mechanical and electrical aberrations (i.e., nonlinear sweeping).In addition, should the voltage sweep direction of the induced potentialbe reversed, a time lag in voltage shift was experienced (i.e.,discontinuous operation). A linear change in voltage within the externalsystem is produced by a logarithmic change in current. Thus, a voltageshift of several tenths of a volt could change the current over severaldecades in magnitude. This linearlogarithmic property required complexswitching equipment to insure even moderately accurate measurement inthe magnitude of current flow. Furthermore, a third problem immediatelyarises. Since the data or readouts were in the linear voltage-amperagemeasurement system, correlating a certain voltage change to a certaincurrent magnitude required a manual plot of volts and-amperes upon logfunction graph paper or other such means. The voltage of the externalsystem can be swept linearly over an extended range (0-10 volts) by thepotentiostatic instrument. The current magnitude can change responsivelyover eight decades in less than 0.5 volts and is very difficult toobtain from linear data whose accuracy is good only for about fourdecades. Thus, the instrument operator was never sure that the voltagesweep information in his readout was directly correlatable to therelated current magnitude. These operational difficulties in priorinstruments have pre- 3 vented the ready and accurate application of theelectrochemical and electroanalytical techniques in evaluating corrosionphenomena, and other related analysis of external systems havingvoltage-current related properties. The present invention is directedtowards an instrument which avoids these problems.

SUMMARY OF THE INVENTION In accordance with this invention, there isprovided a bipolar log converter including a differential inputamplifier having one input receiving a bidirectional current signal,another input connected to circuit common of a DC power supply, and anoutput carrying a voltage representative of the current signal.Rectifier means are series connected and at their common junction areconnected to said input of the differential input amplifier receivingthe current signal. A voltage biasing network provides a current flowthrough the rectifier means for placing them into a conducting conditionfor nonlinear potential-current conversion. An output circuit is seriesconnected across the rectifier means and their common junction connectsto the output of the differential input amplifier thereby forming afeedback loop wherein the current signal passes through one of therectifier means to the amplifiers output for creating a potential in theoutput circuit, and means to provide from said potential a continuouspotential signal representing the current signal whereby the potentialsignal has a polarity representative of the flow direction of thecurrent signal and a magnitude representative of the logarithm of thecurrent signal, and the potential signal changes polarity upon a changein flow direction of the current signal.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical block diagramshowing the various elements comprising the dynamic analyzer of thisinvention;

FIG. 2 is an illustration of a three electrode corrosion cell which hasvoltage-current related properties and four terminals A, B, C and D;

FIG. 3 is a four-electrode electrochemical cell which may be employedfor measurement of conductivity of the liquid in the cell;

FIG. 4 is a schematic of a current limiter employed with the currentsource of FIG. 1;

FIG. 5 is a schematic of a bipolar log converter employed in the presentdynamic analyzer;

FIG. 6 is a schematic of the absolute value circuit employed with thedynamic analyzer of the present invention; and

FIG. 7 is a schematic of the voltage sweep generator employed with thedynamic analyzer of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS In FIG. 1, there is shown in a blockdiagram a preferred embodiment of the dynamic analyzer of the presentinvention. The dynamic analyzer, as identified by legends, is comprisedof an input circuit which includes a pair of terminals A and B and maycontain an isolation amplifier, and output means for producing a firstvoltage signal representative of the potential difference V at theseterminals. This first voltage signal is summed to a second voltagesignal in an error signal amplifier with a linear sweep voltage signalV, from a voltage sweep generator, and if desired, an offset voltagefrom an offset voltage source. The second voltage signal (error signal)drives a power amplifier which passes, bidirectionally, current betweena pair of terminals C and D. The power amplifier may be protected by acurrent limiter. The current I from the power amplifier is monitored bya bipolar log converter having a bipolar potential signal output of iVlog I. Preferably, the bipolar potential signal is applied to anabsolute value circuit which provides a unipolar voltage output signalV= log I. The signal output from either the bipolar log converter orabsolute value circuit is applied to a readout means, such as an X-Yplotter, wherein either signal is directly readable, as log I overseveral decades, in direct coordination and comparison to the linearsweep voltage V For purposes of the present description, the externalsystem may be a three electrode corrosion measurement cell asconnectable to the dynamic analyzer. The corrosion cell 11, as shown byreference to FIG. 2, is formed by a container with electrodes 12, 13 and14 immersed in an aqueous corrodant. The electrodes 12, 13 and 14 willbe described in the functions of reference, test specimen, and third orauxiliary current electrodes, respectively. Electrical conductorsconnect the electrodes 12, 13 and 14 to terminals A, B, and C,respectively. In addition, terminal D also connects to the testelectrode 13. Thus, the cell 11 forms a four terminal external system inwhich terminals A and B are employed for sensing the induced potentialV,, at electrodes 12 and 13 and terminals C and D for passing current(I) through the corrodant and between electrodes 13 and 14. Returningnow to FIG. 1, the terminals A, B, C and D are connected to the terminalblock 16 forming an interface between the cell 11 and the dynamicanalyzer. The terminals A and B connect with an input circuit forsensing the potential V, and having an output means in a voltage loopfor producing a voltage signal representative thereof. An isolationamplifier 17 may be included in the voltage loop for maintaining highimpedance in the input circuit or for other reasons. The voltage loopmay include an isolation amplifier 17, a buffer amplifier 18, an errorsignal amplifier l9, and a source of offset voltage 21 which includes abuffer amplifier 22. An input circuit at high impedance is formed aboutthe terminal block 16 at terminals A and B so that no significantchanges are made to the sensed potential V,, at cell 11.

The isolation amplifier 17 may be of any conventional type having highinput impedance and common mode rejection. The isolation amplifier 17 isa dual input-output differential amplifier having positive and negativeinputs 23 and 24 connected to the reference and test electrodes 12 and13, respectively. The amplifier 17 may have a component gain of 50,000or greater, but the components of the input circuit associated with suchamplifier 17 adjust the input-output circuit gain to approximatelyunity. This arrangement of the isolation amplifier 17 provides an inputcircuit which has exceedingly high impedance input, stability and a highcommon mode of rejection. The isolation amplifier 17 has the usualconnections at terminals B+, B- and common (CM). Power supply 28provides operative power for the present dynamic analyzer.

The positive and negative outputs 26 and 27 of the isolation amplifier17 are connected in the voltage loop to the buffer amplifier 18, theerror signal amplifier 19, the offset voltage source 21, the bufferamplifier 22, and circuit common. The output 26 is matched through thebuffer amplifier 18 to be a relatively low impedance at the error signalamplifier 19. The error signal amplifier 19 may be of conventionaldesign and with the usual connections to the power supply 28.Preferably, the amplifier 19 is a differential amplifier for purposes ofinsuring a tracking of the input voltage V,, to the isolation amplifier17 with the sweeping voltage V,,'. The error signal amplifier 19 has afirst input 31 connected to its output 32 through a feedback network ofresistors 33 and 34 with a shunting capacitor 36 providing a modifiedtime constant function. With this feedback arrangement, the amplifier 19has a substantially low gain at high frequency for stability purposes.The voltage sweep signal V, is applied to the second input 37 of theamplifier 19. Thus, input 31 receives a voltage signal V correspondingto the input voltage between the inputs 23 and 24 of the isolationamplifier 17. The voltage sweep signal V, at input 37 is summed with thepotential V and the error signal amplifier 19 produces a voltage signalat its output 32 which is a function of the difference between thesevoltage signals at the inputs 31 and 37. This voltage signal is theerror signal voltage which is applied to an input 41 of the poweramplifier 42. v

The error signal voltage at the input 41 drives the power amplifier 42to provide in output 43 an output current I which flows betweenterminals C and D and theelectrodes 13 and 14 in cell 11. From terminalD, current I returns to circuit common through the range resistornetwork 173 and through input resistor 176 to input 174. The voltage Vat terminal D and at input of buffer amplifier 44 is thereforeproportional to I. Amplifier 44 has an output V applied to currentlimiter 46. Current limiter 46 controls input 47 of power amplifier 42so that output 43 can never exceed I of the cell at terminal D. Thepower amplifier 42 produces the current I in output 43 and causespolarization potential V,, to be formed between electrodes 12 and 13.This potential appears in the voltage loop at input 31 of the errorsignal amplifier 19 and summed with the sweep voltage V, appearing atinput 37. Any difference in voltage between inputs 31 and 37 producesthe error signal at the input 41 of the power amplifier 42 which adjuststhe current I so that the induced potential V appearing at inputs 23 and24 of the isolation amplifier 17 is precisely equal to, or tracks, thesweep voltage V,,'.

In some cases, it is desired to have the potential V between theelectrodes 12 and 13 summed with a fixed potential (offset) at theinputs 23 and 24. It may be desired to determine a corrosion environmentin cell 1 l where the induced potential V at electrodes 12 and 13 ischanged by some small fixed potential, for example, 100 millivolts. Forthis purpose, the output 27 of the isolation amplifier 17 connects tocircuit common through the offset voltage source 21 and the bufferamplifier 22. The buffer amplifier 22 merely provides an impedanceadjustment and isolation between the offset voltage source 21 and theoutput 27. The offset voltage source 21 may be a high impedance voltagedivider network connected to the power supply 28 with an adjustablevoltage control indicated by the rheostat 29. The offset voltage is aselected steady state value of positive or negative polarity. Thisoffset voltage is summed precisely with the voltage V appearing in theoutput 26, and the resultant voltage appears at the input 31 of theerror signal amplifier 19. As a result, this circuit functionsidentically as previously described but with the potential V, betweenthe electrodes 12 and 13 now tracking the sweep voltage V,,' by a fixedoffset voltage magnitude provided by the offset voltage source 21 (V,, Vi offset voltage).

Switch 48 and its resistors 49 and 51 are for applying a limited smallcurrent to the external cell 11 to determine the offset voltagenecessary to bring to zero this small current. At this point, thepotential at terminals A and B equals the offset voltage. In order toprotect the cell 11 from excessive current surges, the current limiter46 effects an adjustment of the current flow returned to circuit commonby applying a protective bias voltage to the input 47. The currentlimiter 46 is buffered from the current loop by buffer amplifier 44.

In FIG. 4, the current limiter 46 comprises differential input amplifier56 and 57. The amplifier 56 has a first input 58 connected through aresistance 59 to the output of buffer amplifier 44 and the other input61 is at circuit common. The output 62 of the amplifier 56 connects in afeedback loop containing fixed and variable resistances 63 and 64,respectively. A current derived fromV proportional to I passes through aresistor 59 to input 58 serving as the summing point. The invertingamplifier 56 holds input 58 at circuit common. The resistance 64adjustment establishes the current I level at'which amplifier 56saturates. At current I less than maximum value, bidirectional,amplifier 56 holds input 58 at very near circuit common. When amplifier56 issaturated, input 58 moves voltagewise from circuit common.Resistance 66 provides a current into summing point at input 58 whichcauses amplifier 56 to effectively saturate at some magnitudes ofcurrent I maximum for both directions of I maximum. The amplifier 57connects with input 67 through a resistor 68 to the input 58 of theamplifier 56. The input 69 of the amplifier 57.connects to its output 71through a feedback resistor 72 and shunting capacitor 73. The input 69also connects through a resistance 74 to circuit common The output 71connects to the input 47 of the power amplifier 42 and through thevoltage divider resistance 49 to circuit common. The amplifier 57becomes active only when amplifier 56 reaches saturation. The poweramplifier 42 is limited to that current output which is necessary tosaturate amplifier 56 and this function is selected by resistors 63 and64.

Returning now to FIG. 1, the linear sweep voltage V,, applied to theinput 37 of the error signal amplifier 19 is obtained from a voltagesweep generator 81. The voltage sweep generator 81 has one output 82 atcircuit common and a second output 83 connects to the input 37 of errorsignal amplifier 19. The input 37 is shunted to circuit common through aresistor 84. The resistor 84 is part of a common mode rejection networkwhich also usually has the same effect upon current flows to circuitcommon as the resistor 45 associated with the power amplifier 42. Thevoltage sweep generator 81 in a preferred embodiment produces atriangular wave sweep voltage which, as illustrated diagrammatically,ramps linearly between voltage magnitudes V V At any particular instant,this triangular sweep voltage has a magnitude and rate of change whichmay be indicated b V The: voltage sweep generator 81 is adjustable as toboth magnitudes and polarity of V and V and also in the rate of sweep.Further, the voltage sweep generator 81 can be adjusted as to sweep onlyfrom voltage V to V,, or V to V or any portion thereof, or to sweepcontinuously between these two values over any practical time limit. Forexample, sweep rate may be the completion of one full triangular wave in24 hours or in minutes. Irrespective of the sweeping rate, the output ofthe voltage sweep generator 81 is linear.

In particular, the voltage sweep generator 81 of the present inventioncan be arranged to have voltage sweep rate limits between 0.01 volts perhour to 1000 volts per hour; it can hold any particular set voltage V,,;or it can sweep continuously between the voltage limits V and V or fromzero to either one or the other of these voltage magnitudes. Thus, thevoltage sweep generator 81 can produce a linear sweep voltage signal V,from a first magnitude V to a second magnitude V and preferably itprovides a triangular wave sweeping voltage signal.

Referring now to FIG. 7, the voltage sweep generator 81 is shown incircuitry detail. The generator 81 includes an integrator having anoutput circuit carrying the linear sweep voltage signal produced inresponse to a control current signal. A control network provides thecontrol current signal upon receipt of a control signal voltage of fixedmagnitude and positive or negative in polarity. A sweep referencevoltage source provides first and second reference voltages. The linearsweep signal is correlated to the reference voltages in a comparatorwhich generates switching signals at the sweep signal reaching each ofthe reference voltages. The switching signals are applied to a bistableamplifier which produces the control signal voltage of fixed magnitudebut alternates in positive and negative polarity. The polarity of thecontrol signal voltage determines whether the linear sweep voltage rampsup or down voltagewise.

More particularly, and integrator 91 receives a current control signaland provides responsively in an output circuit 92 a linear sweep voltagesignal. For this purpose, the integrator 91 can be a differential inputamplifier having the usual connections with the power supply 28, a firstinput 93 receives the control current signal, and a second input 94 isat circuit common. The output 92 connects through voltage dividingresistors 96 and 98 to a voltage follower 97. The capacitor 99 connectsbetween the input 93 and output 92 for providing the integratingfunction. The input 94 connects to circuit common. As the potential atinput 93 tends to change through the flow of current through resistor101 into the capacitor 99, the output 92 swings voltagewise to holdinput 93 at circuit common. If the current at the input 93 is of uniformmagnitude, the voltage change at the output 92 is a linear function. Forexample, the control current signal flowing through resistance 101 tothe input 93 is indicated by i As this current charges the capacitor 99,the output 92 changes in a negative direction with a linear function.Conversely, a current flowing from input 93 through the protectiveresistor 101 is indicated by i and produces a positive linear voltagechange at the output 92. The conductor 103 from the voltage follower 97is applied to terminal 104 and by suitable connection to the input 37 ofthe error signal amplifier 19. The terminal 111 of the voltage sweepgenerator 81 is at circuit common. The terminals 104 and 111 carry thelinear sweep voltage V, represented as triangular wave 106 which rampsbetween voltage magnitudes V and V The directional current flow of thecontrol current signal i or i in the resistor 101 determines whether thelinear sweep voltage V,, is increasing or decreasing in magnitude.Therefore, the control current signal is derived from a relativelystable source and, in the present voltage sweep generator 81, a controlnetwork 112 is employed for this purpose.

The control network 112 is comprised ofa conductor 113 carrying a finitepositive or negative control signal voltage magnitude. The source ofthis control signal voltage will be described hereinafter. The controlsignal voltage is applied through movable switch segment 114 to aplurality of contacts 116, 117, 118 and 119. These contacts areconnected into a resistance network formed of fixed resistors 121 and122, a variable resistance 123, and connected through a seriesresistance 124 to the resistor 101 at input 93 of the integrator 91.These network resistances in the control network 112 provide a stableand selectable impedance divider whereby the positive or negativecontrol signal voltage is converted into a finite control current signalwhich flows through the resistance 101. The several switch controlsprovide for selecting the rate of change of the linear sweep voltage.For example, the switch at contact 116 selects a sweep rate of 0 tovolts per hour; at contact 117, a sweep rate of 0 to 10 volts per hour;and contacts 118 and 119 provide functions in the analyzer, which willbe described in greater detail hereafter, of Hold at a selected V,,' andHome to V or V Sweep rates at contacts 116 and 117 are determined byvariable resistor 123.

The control signal voltage applied to the control network 112 may beeither positive or negative in polarity to determine, respectively,whether the linear sweep voltage is increasing or decreasing inmagnitude. In accordance with the preferred embodiment of thisinvention, the control signal voltage is made alternately positive andnegative in polarity so as to produce a triangular linear sweep voltagesignal 106 in the output 92 of the integrator 91. The control network112 in conjunction with the control voltage signal provides forregulating the magnitude and direction of current flow of the controlcurrent signal through the resistor 101, thereby determining the rate ofincreasing or decreasing of voltage in the linear sweep voltage signalin output 92.

The control voltage signal in the conductor 113 is produced in a novelfashion with alternate changes in polarity but of a preset fixed andstable magnitude. For example, the control signal voltage may be apositive or a negative 10 volts. The control signal voltage is providedby a bistable amplifier 126 which has the usual connections to the powersupply 28. Preferably, the bistable amplifier 126 is a differentialinput amplifier having a component gain of about 50,000. However, apositive feedback circuit of resistors 127 and 128 between its output129 and first input 131 provides the feedback necessary to obtain abistable operation. The voltage divider of resistors and 132 provides aportion of the integrator output 92 voltage at the input 131 of thebistable amplifier 126. The bistable amplifier 126 switches fromsaturation from one polarity voltage limit to the other polarity voltagelimit when the voltage difference becomes zero between inputs 131 and136. This operation results from positive feedback to input 131 from theoutput 129 and the high internal gain of amplifier 126. The resistors133 and 134 compensate for any difference in the saturationcharacteristic of the bistable amplifier 126 about zero output voltage.The bistable amplifier 126, when correctly adjusted, causes the controlsignal voltage to swing to and remain at one of two equal magnitudes ofvoltage but opposite in polarity. This function is indi- 9 cated by thegraphic display 137 as shifting between +V and V about a zero voltagemagnitude.

The bistable amplifier 126 is switched from one to the other polaritycontrol voltage signal by successive switching signals 138 applied toinput 136. The input 136 connects to circuit common by a resistor 139and receives the switching signal shown in the graphic display 138. Eachsuccessive switching signal upon the input 136 causes the output 129 tomove from one to the other polarity of the control signal voltage. Thebistable amplifier 126 switches its output 129 between absolute andstable finite magnitudes equal but opposite in polarity upon receipt ofsuccessive switching signals at the input 136 irrespective of themagnitudes of such switching signal.

The switching signals can be of opposite polarity as indicated by thediagrammatic representation 138. The switching signals may be of anyduration or of uniform or nonumiform magnitude as long as they aresufficient in both characteristics of polarity and voltage magnitude tocause the inputs 131 and 136 of the amplifier 126 to become equalpotentialwise. With the proper switching signal, the positive feedbackabout the amplifier 126 causes the output 129 to shift so that thecontrol signal voltage is at one of the limits +V and V set by thevariable resistance 133. Immediately after shifting directionallythrough zero, the positive feedback of the bistable amplifier 126 holdsthe output 129 to maintain such voltage limit, l-V or V, until the nextsucceeding switching signal of opposite polarity. For example, the firstswitching signal in the diagram 138 is indicated as positive going. Thiscorresponds to the control signal voltage in the diagram 137 beingswitched in a negative direction to the limit of negative polarity (-V).The next succeeding switching signal is negative going and causes thecontrol signal voltage 137 to be switched to the limit of positivepolarity, +V. The control signal voltage remains at each such magnitudeand polarity until the next succeeding switching signal of oppositepolarity.

In the voltage sweep generator 81, the switching signals occur in exacttiming to the linear sweep voltage between terminals 104 and 111reaching the magnitudes V and V respectively. As the triangular wavesweep voltage reaches the magnitude V the switching signal may be ofnegative polarity, and upon reaching the magnitude V may be of positivepolarity. For this p'urpose, the voltage sweep generator 81 includes acomparator for determining when the triangular wave sweep voltagereaches one of the magnitudes V or V and produces the switching signalsat the precise timing when these values are reached. For this purpose,the comparator samples the linear sweep voltage signal at the terminal104 and compares this sweep voltage sig nal with first and secondreference voltages V and V respectively, defining particular magnitudesof the sweep voltage limits V and V as indicated in the graphic display106. Precisely as each of the sweep voltage magnitudes, V and V arereached in reference respectively to the first and second referencevoltages, successive switching signals occur with opposite polarities.

The first and second reference voltages V and V are obtained from anysuitable source such as a resistance divider network connected to thepower supply 28. This resistance network includes dropping resistors 141and 142 which connect across paralleled potentiometers 143 and 144. Thevalue of these resistances are so arranged that the first and secondreferences voltages V, and V,, (relative to circuit common) appear atthe movable contacts 146 and 147 on these potentiometers. With thearrangement shown, V and V may be of any magnitude and any polarityrelative to one another. For example, V and V may be of equal magnitudebut opposite in polarity. Alternatively, V and V may be both positive invalue, but of different magnitudes. Alternatively, V and V may benegative in value and of different magnitudes. Also, V and V may betaken at either one of the movable contacts 146 and 147. In order toprovide an instant reversal of sweep direction, the resistor network isinterconnected through a momentary double-pole, triple-throw switch 148.The switch 148 at terminal 148a is the normal switch position where bothreference voltages V and V are available at the movable contacts 146 and147. Placing the switch 148 into the position 148b substitutes newreference voltage limits that are both to one side of V,,' voltagewise.Moving the switch 148 to the position 1486 substitutes new referencevoltage limits that are both to the other side of V,,' voltagewise.Thus, positions 148b and 1480 permit sweep direction reversal. Returningthe switch to position 148a returns the voltage sweep limits to V and VIf V,, resides between V and V the sweep direction will not be reversed.Thus, there is a feature of selected sweep direction reversal in thepresent voltage sweep generator without disturbing the voltage sweeplimits V and V The comparator has differential input amplifiers 151 and152 with their inputs receiving the first and second reference voltagesand the sweep voltage signal at the terminal 104. The amplifiers 151 and152 should have relatively high component gains of approximately 50,000and are adapted with zero input voltage to move from one saturated stateto the other saturated state of opposite polarity and then returns intothe first saturated state. With this arrangement, high sensitivity tosmall potential differences between the voltage limits V and V of thelinear sweep voltage signal can be readily compared to the first andsecond reference voltages. The amplifier 151 has a first input 153connected to the movable contact 147 to receive the first referencevoltage. The amplifier 152 has an input 154 connected to the movablecontact 146 to receive the second reference voltage. The remaininginputs 156 and 157 of these amplifiers are connected together and to theterminal 104 to receive the linear sweep voltage signal for comparisonpurposes. As the sweep voltage magnitudes V, and V are approachedclosely by the linear sweep voltage at the terminal 104, the voltagesbetween the inputs of one of the amplifiers 151 and 152 approach a zerovoltage signal differential and this amplifiers output swings from onesaturated state to the opposite polarity saturated state. The outputs158 or 159 of the amplifiers 151 and 152, respectively, move suddenlyvoltagewise to preset magnitudes that are determined by an outputresistor-diode network. As a result, the active amplifier reverses stateand returns to its original saturated state. For this purpose, theoutputs 158 and 159 connect through series resistors 161 and 162, andrectifiers 163 and 164, to a conductor 169 which connects with the input136 of the bistable amplifier 126. The rectifiers 163 and 164 are seriesconnected between the outputs 158 and 159. When the voltage magnitudelimits V, or V for the linear sweep voltage signal reside between thefirst or second reference voltages V and V the amplifiers 151 and 152have their outputs 158 and 159 shifted voltagewise in such a manner thatthe rectifiers 163 and 164 are both biased into a conducting ornonconducting state. When the linear sweep voltage signal at theterminal 104 reaches one of the first or second voltage magnitudes V, orV one of the rectifiers is biased to a conducting state and the other ofthe rectifiers is biased into a nonconducting state whereby a voltagesignal produces the switching signal on conductor 169. These switchingsignals alternate in polarity, but each of them occurs precisely as thelinear sweep voltage reaches one of the first or second voltagemagnitudes V or V The resistances and rectifiers in the outputs of thecomparator amplifiers 151 and 152 are a logic circuit for generating theswitching signals of alternate polarity in succession.

The comparator with the switch 114 in the Home position 119 may beemployed for causing the linear sweep voltage signal to go to either ofreference voltage sweep limits V or V For this purpose, a switch 166 isemployed in a single-pole, triple-throw function. The switch in position166a connects through a load resistor 167 to the output 158, and inposition 166b connects through a resistor 168 to the output 159, and incentral position 1660 connects to circuit common. In the normaloperating position 166e, the voltage sweep generator 81 can be zeroedunder static conditions at the Hold position of switch 114 at contact118. In the position 166a, the reference sweep voltage V will appear atterminal 104. In position 166b, the reference sweep voltage V appears atthe terminal 104. In position 166e, zero voltage appears at terminal104. During normal operation of the instrument with the switch 114 atthe contacts 116 or 117, the switch 166 is inactive. However, the switch114 at the contact 119 places the sweep voltage generator 81 into theHome function. Also, switch 114a in Home removes the positive feedbackfrom amplifier 126 by shorting the juncture of resistors 127 and 128 tocircuit common. The switch 114 at contact 118 Hold will hold the sweepvoltage signal at its instant magnitude V,,. The switch 114 also hassections 114a and l14b which function exactly as previously described.In the Home position, the switch 114b connects to the switch 166 and theconductor 169, and with input 136 of the bistable amplifier 126.

The comparator in the present voltage sweep generator 81 is of greatadvantage in providing several unique sweep generator functions whichhave been heretofore described and can be used separate and apart fromthe dynamic analyzer. The control current signal from control network 112 that is applied to the integrator 91 is always stable and of a presetmagnitude even though it undergoes alternate directional changes in flowthrough the resistor 101. The resistor 139 returns the logic circuit tocircuit common.

In the present voltage sweep generator 81, placing the switch 114 intothe Hold function terminates the application of the control signalvoltage on the conductor 113 into the control network 112. At such time,the integrator 91 ceases to receive a control signal current through theresistor 101 and integration stops within capacitor 99. As a result, thelinear sweep voltage V,,' is held at the potential last achieved beforethe switch 114 was moved into the Hold position. Thus, a fixed sweepvoltage V, is available at terminal 104 for reference setting or forother purposes.

Referring again to FIG. 1, the linear sweep voltage from the voltagesweep generator 81 is applied to the input 37 of the error signalamplifier. As a result, the

power amplifier 42 produces a current flow between terminals C and D,and concomitantly between electrodes 13 and 14 of the external system,for inducing a polarizing potential V,, between electrodes 12 and 13,which potential V appears at terminals A and B and across inputs 23 and24 of the isolation amplifier 17. As previously mentioned, the potentialat the inputs of the isolation amplifier 17 can be precisely the linearsweep voltage. If desired, the polarization potential at inputs 23 and24 can be the linear sweep voltage V,, summed with an offset voltagefrom the offset voltage source 21. In other words, the polarizationpotential at teminals A and B tracks the linear sweep voltage, and anyoffset voltage. The current flow I between the terminals C and D in theexternal system may be a logarithmic function of the polarizationpotential V present at the terminals A and B. Thus, if the voltage sweepgenerator 81 produces a linear sweep voltage signal, the current I willhave a logarithmic function in time. A readout comparison between alogarithmic current and a linear sweep voltage signal produces greatdifficulties not only in a display or correlation, but also indetermining corresponding multidecade ranging of extended time ofcurrent and voltage magnitudes in calibration and for other purposes.

The present dynamic analyzer employs a unique method of correlating andcalibrating the log current and the linear sweep voltage signalfunctions. For this purpose, a bipolar log converter 71 senses thisoutput current I between terminals C and D and produces a bipolarpotential signal iV= log I. The potential signal has a polarityrepresentative of the flow direction of the output current. Also, thepotential signal has a magnitude representative of the logarithm of themagnitude of the output current. Furthermore, the potential signalchanges polarity upon a change in the flow direction of the outputcurrent. The bipolar log converter 171 has one input connected tocircuit common and a current sensing input 174 connected to a rangesetting resistance network 173 and a current dividing resistor 176.Thus, the current signal at the input 174 is proportional in a fixedratio to the current I flowing between terminals C and D.

In FIG. 5, there is illustrated circuit forming the bipolar logconverter 171. The bipolar log converter 171 includes a differentialinput amplifier 181 having one input 182 connected to the input 172. Theother input 183 connects through a resistance 184 to circuit common. Asa result, the current signal I appears at the input 182 (current summingpoint) of the amplifier 181. The amplifier 181 has an output 186 whichmoves voltagewise in specific correlation to the current signal I at theinput 182. Rectifiers are connected in series with their common punctureconnecting to the input:

182 of the amplifier I81 and with their other terminals connectingacross a voltage biasing network for placing the rectifiers into aconducting state. These rectifiers may take any suitable configurationfor producing a nonlinear potential-to-current conversion. For example,a current flowing through either of such rectifiers produces a linearsignal voltage corresponding to the logarithm of the current magnitude.An output circuit connects in series across the rectifiers and theircommon junction is connected to the output 186 of amplifier 181 forminga feedback loop wherein the signal current passes through one rectifierto the output 186, thereby creating a potential signal in the outputcircuit. This potential is employed for providing the potential signalrepresenting the logarithm of current 1.

Many types of such rectifiers are known, but for purposes of thispresent invention, it is preferred that the rectifiers be provided bytransistors having collector-emitter junctions arranged to perform therectifier function. Preferably for this purpose, a pair of rectifiersare formed by PNP transistor 187 and NPN transistor 188 with their basesconnected together and to circuit common, and with resistor 184 inseries with input 183 of amplifier 181. The collectors of thesetransistors are connected together and to the input 182 of the amplifier181. The emitters 189 and 191 connect to a voltage biasing network whichprovides a small but steady state current flow from any suitable sourcesuch as the 3+ and B- terminals of the power supply 28 for biasing thetransistors 187 and 188 into a conducting state, thereby producing thementioned nonlinear potential-current conversion. More particularly, theemitter 189 connects through a resistor 192 to the B+ terminal of thepower supply 28. The emitter 191 connects through a resistor 193 to theB- terminal of the power supply 28'. The voltage biasing network maytake any operable form such as a pair of batteries series connected withthe rectifiers and thereby forming a voltage bridge. Preferably thevoltage bridge has the following forms. A pair of diodes 194 and 196 areconnected in series between the emitters 189 and 191 and the resistors192 and 193. The junction 197 between the diodes 194 and 196 connectsthrough a current-limiting resistor 198 to the outut 186 of theamplifier 181. The voltage biasing network may include a four-arm bridgecomprising resistors 201 and 202 and resistors 203 and 204 in serieswith the first mentioned resistors. It is preferred to employ theresistors in such a bridge in order to provide a well-controlled currentdivider so that the current which places the transistors into conductionis only a small portion of the current (l-2 nanoamperes) which flowsthrough this four-arm bridge and the smallest value of current I. Moreparticularly, it is preferred that the current flow through thecollector-emitter junctions of the transistors relative to the currentflows through the four-arm resistance bridge and current I is in a ratioof at least 1 to 1,000 and preferably a higher ratio of l to 100,000 isemployed for greater stability. Thus, a small biasing voltage isdeveloped across each transistor.

The transistors 187 and 188 are arranged with their bases at circuitcommon so that no significant current can flow between their baseconnections and the collectors and emitters. As a result, all signalcurrent must flow through the collector-emitter junctions in a rectifierfunction since the transistors are in a forward conducting state. Thecurrent I flow through the collectoremitter junctions produces aconversion into a corresponding linear potential signal V which is thelogarithm of the magnitude of current 1. More particularly, currentflows about the summing point at the input 182 and is matched by equalcounter current flows to the summing point from the feedback connectionto the collectors of the transistors 187 and 188. The output 186 of theamplifier 181 shifts voltagewise in response to the current I appearingat the summing point of input 182. Point 197 moves responsively involtage and an equal current flows between input 182 and the commoncollector connection of the transistors, through one of thecollector-emitter junctions, is gated through one of the diodes 194 or196 and flows through the resistor 198 to or from the output 186. Thediodes 194 and 196 selectively gate the current signal I through one ofthe collector-emitter junctions in a feedback loop for conversion ofthis current signal, in a nonlinear manner, into a potential signal. Thepolarity of the potential signal is determined by which transistor 187or 188 has conducted the current signal and is summed with the biasvoltages across the conducting transistors. Only one transistor conductscurrent I at any time as the current in the feedback loop flows to andfrom the output 186 of the amplifier 181. Thus, the polarity of thelinear potential signal created by current rectification at acollector-emitter junction of each transistor reflects the directionalflow characteristic of the current I which passes between terminals Cand D to the external system. As a result of the potential signalcreated at one of the transistors 187 or 188, the potential at point 197between the resistors 192 and 193 shifts voltagewise. This potentialsignal summed with the biasing voltages developed across the transistorsapproaches zero as the current I nears zero during direction reversal.As a result, the potential signal V changes substantially linearly overthis small range of current magnitudes each side of zero current. Theresistor network 203 and 204 responsively producesat their juncture 205a corresponding potential signal relative to circuit common. Thispotential signal corresponds in polarity to the current signal I at thecollector-emitter junctions of the transistors 187 and 188. Thus, thislinear potential signal at juncture 205 of the resistors 203 and 204 iscorrelated by magnitude and polarity in the relationship iV= log I.

Obviously,'the most optimum functioning of the transistors 187 and 188,the amplifier 181 and the other components of this log convertercircuit, can require temperature correction. If such feature is desired,a voltage follower differential amplifier 209 can be employed foradjusting the potential signal in its output 213 to compensate for anygain errors induced by temperature variations. The amplifier 209 has afirst input 208 connected to the juncture 205 of the resistance bridge.The other input 211 of the amplifier 209 connects through aresistor-capacitor feedback network 212 to the output 213. A variableresistance 214 connects between the input 211 and circuit common. Theresistance 214 is selected with a temperature coefficient so as toadjust the gain of the amplifier 209 and thereby correct temperaturewisethe potential signal at output 213. This potential signal at the output213 is indicated at iV= log I. Thus, the bipolar log converter 171produces a bipolar conversion in the current I into a potential signal,and maintains the exact and precise relationship thereto irrespective ofdecades of change in the current I.

Internal temperature correction can be applied to the transistors 187and 188, if desired. A complementary transistor can be mounted on eachof the chips bearing the transistors 187 and 188. For example, a PNPtransistor 216 is mounted on the chip with the transistor 187. A NPNtransistor 217 is mounted on the chip with the transistor 188. Thetransistors 216 and 217 are connected with their collectors connectedtogether and to the junction between the resistors 201 and 202. Theemitters of the transistors 216 and 217 are connected emitter-emitterwith transistors 187 and 188. The base of the transistor 216 connects tothe junction between resistors 202 and 204. The base of the transistor217 connects to the junction between resistors 201 and 203. Thus, thetransistors 216 and 217 share common chips with the transistors 187 and188. However, the transistors 216 and 217 are connected in reverse basesignal to the junction of the first and third and second and fourthresistances of the bridge. As a result, the collector-emitter junctionsof the transistors 216 and 217 provide a low impedance current path. Atemperature change in any one of the collector-emitter junctionsproduces a corrective voltage in the other transis tor on the same chip.As a result, no temperatureinduced current variation can effect thenonlinear potential-to-current conversion at the collector-emitterjunctions of transistors. The collector-emitter junctions of thetransistors 216 and 217 provide a low impedance path for currentcompensation in the first and second resistances 201 or 202. The currentcompensates for any voltage shift in these resistances during thenonlinear potential-to-current conversion at the collectoremitterjunctions of the transistors 187 and 188. The transistors 216 and 217provide for temperature compensation in the low impedance arms of thebridge, which arms sustain a major part of current flow while limitingthe amount of current flow placing the transistors 187 and 188 into aconducting state at their collector-emitter junctions. The transistors216 and 217 provide an exacting current control for insuring a steadycurrent flow through the transistors 187 and 188 irrespective oftemperature changes internally within these transistors or levels of thecurrent I.

The amplifier 181 has a capacitor 218 in its feedback circuit. Thiscapacitor provides the amplifier 181 with an active filter involtage-to-current application to insure absolute tracking betweenvoltage swing at the output 186 relative to the current signal Iappearing at the input 182. The amplifier 181 with this arrangement hasexcellent low current noise bypass relative to the current I flowingthrough the transistors 187 and 188. However, as the current I changesdirection and the potential signal V at the output 213 of the voltagefollower amplifier 209 passes through zero, a nonlinearity may occur atzero crossing under a high rate of change in the current I. This problemcan be reduced by providing the bipolar log converter 171 with acapacitance cancellation system. For this purpose, an invertingamplifier 221 is connected with one input to circuit common, and theother input 222 to the output 186 of the amplifier 181. The amplifier221 can be a conventional differential input type with a feedback loopformed of fixed resistances 215 and 223, and a voltage settingpotentiometer 224. A coupling capacitor 226 connects the potentiometer224 to the input 182 of the amplifier 181. The resistance in thefeedback loop of the amplifier 221, and the ratio of the capacitor 226to the compacitor 218, are selected so that the amplifier 221 saturatesto remove the effect of capacitor 226 when output 186 swings voltagewisei1 voltage. A rapid change at zero crossing in polarity of the potentialsignal at the output 186 of the amplifier 181 impresses a voltage acrosspotentiometer 224. The amplifier 221 effectively removes the capacitiveeffect of the capacitors 218 and 226 during this interval. The amplifier221 does not have any significant effect except during this interval onthe amplifier 181. Thus, the capacitor 218 is made effectively of valueto permit a uniform rate of change by action of amplifier 221. Otherarrangements for providing a capacitance eliminator result may beemployed. Alternatively, the current change may be compensated bychanging the values of the capacitor 218 in the feedback circuit of theamplifier 181.

The bipolar log converter 171 operates in a continuous manner andconverts the current I at the input 182 into a bipolar potential signalat the output 213. The current I can change through several decades, forexample, four decades. The current may also change direction andcontinue to change for an additional four decades in a reverse flowdirection. The log converter 171 of the present invention produces acorresponding bipolar potential signal at the output 213 which is linearin function and precisely tracks the particular current magnitudes uponwhich logarithmic conversion has occurred. Thus, the potential signal atterminal 213 can be applied to a linear scale recorder. A semilog cyclescale permits a direct readout of the original current I even with zerocrossing. The problems of unipolar log conversion and decade-spanningranges of conversion current are avoided by use of the novel bipolar logconverter employed in the present dynamic analyzer. Therefore, the logconverter can be used in other applications for the conversion :V log I.

The bipolar log converter 171 may be employed with a readout meanshaving semilog cycle scales above and below a zero center scale setting.However, it is preferred for continual recording and direct displaypurposes of comparison of log I to the linear sweep voltage applied tothe dynamic analyzer to employ another unique element of the presentinvention. This element is an absolute value circuit 231 illustrated inFIG. 1 relative to the remaining elements of the dynamic analyzer. Theabsolute value circuit 231 receives the potential signal, iV log I, andconverts this bipolar potential signal into a unipolar potential, V, logI, which also has zero crossing capabilities without distortion. Moreparticularly, the absolute value circuit 231 receives the potentialsignal from output 213 and produces a single polarity voltage(corresponding to all potential signals irrespective to whether they arepositive or negative) and provides such single potential signal withabsolute linearity. As a result, the unipolar potential signal can begraphically displayed on the same semilog cycle graph scale wherein zerolog current I is at one extreme margin. The significance of such displayresult may be appreciated in FIG. 1 by the readout device 232 whichprovides for comparison of the linear sweep voltage signal V,,' with thepotential signal V log I. it is not necessary for the absolute valuecircuit 231 to be employed, but it is preferred to do so. In sucharrangement, the voltage output of the absolute value circuit 231 isapplied to a suitable readout device 232 which may be a X-Y plotter. Forexample, the absolute value circuit output V= log I may be the X axisand the linear sweep voltage signal V,, may be the Y axis on the X-Yplotter. The X-Y plotter has a linear scale for the Y axis which is involtage per scale division. The voltage V, has a positive and negativevalue to the left and right margins with a center zero potentialposition. Along the X axis, the lower extremity represents zero currentI and proceeds upwardly in log cycle scale for a selected number ofdecades, which may be for example five decades.

The readout device 232 contains a graphic curve 233 which isrepresentative of an actual recording produced by the dynamic analyzerof the present invention where the triangular sweep voltage signal V,,'was employed between two voltage magnitudes, one being positive, theother being negative and passing through zero. Consider the recording tobegin at the point 234 as a sweeping voltage signal (anodic) decreasefrom a negative value towards zero along the curve portion 236. Wheneverthe sweep voltage signal passes through zero at the point 237, thevoltage signal reverses and the curve portion 238 is followed until thepoint 239 is reached. Again, the sweep voltage signal (cathodic) beginsto change direction and the curve portion 241 is followed until point242 is reached at the other magnitude of sweeping voltage signal. Then,the sweeping voltage signal begins to return to its original value alongcurve portion 243 of the curve display 233. During this triangular wavesweep, it will be apparent that the linear sweep voltage signal passestwice through zero and the current I flowing between the terminals C andD also passes through zero twice. However, all of the display is shownupwardly from zero along the X axis. This log current function V= log Imay be read directly on a semilog scale, and also in direct comparisonto any value of sweep voltage signal V along the Y axis. Such graphicdisplays of electrochemical and electroanalytic phenomenon cannot beobtained in any known prior instruments.

The curve 233 reflects the anodic phenomena in portions 236 and 243 andcathodic phenomena in curve portions 241 and 238. Anodic conditionsrequire a current flow between terminals C and D in one direction, andcathodic conditions require current flow in the opposite direction atthese terminals. The present dynamic analyzer acting upon a threeelectrode cell as shown in FIG. 3 can produce such actual recording incurve 233 without resetting during continuous operation. It will producesuch recording during relatively slow or very fast sweep rates in thelinear sweep voltage signals provided by the voltage sweep generator 81.

The curve 233 indicates to the electrochemist the phenomena occurring bycorrosion about the test electrode 13. For any given linear sweepvoltage signals, the magnitude of current I flowing between terminals Cand D can be correlated to the corrosion occurring (both anodically andcathodically) over long or short periods of time, and in completelyunattended operation. This analysis result remains whether the linearsweep voltage V,, passes through zero voltage, or where all sweepvoltage is positive or negative in polarity. Additionally, the readoutdevice 232 operates unattended, can span any number of decades of changein the current I and will retrace successively the curve 233.

Referring to FIG. 6, the absolute value circuit 231 is shown in detail.The absolute value circuit includes a unity gain amplifier 251 and avoltage follower amplifier 252, both of which may be differential inputamplifiers connected to the power supply 28. The amplifier 251 has oneinput at circuit common and another input 253 connected into an inputcircuit 254 which receives the potential signal :V log I from thebipolar log converter 171 at output 213. Both amplifiers operate in aninverting mode. A potentiometer 256 connects between first and secondresistances 257 and 258. The resistor 257 connect to the input 254 andresistor 266. The resistor 258 is connected through a diode 261 to theoutput 262 of the amplifier 251. A feedback loop comprised of a resistor263 and diode 264 connects between the input 253 and the output 262 ofthe amplifier 251. The diode 261 connects through third and fourthresistors 266 and 267 to the input 254 and to the amplifier 252 at itsinput 268. The junction 269 of the resistors 266 and 267 is a currentsumming point. The input 268 of the amplifier 252 connects to B+ of thepower supply 28 through a potentiometer 271. The amplifier 252 is avoltage follower having a feedback loop comprised of resistor 272 andvariable resistor 273. The output to the readout device 232 is takenbetween terminal 274 connected to the variable arm of the potentiometer273 and terminal 276 connected to circuit common. Adjustment of thepotentiometer 271 provides for precisely setting the decade range of Vlog I relative to the actual voltage input to the X axis of the readoutdevice 232 for a given current decade readout. The circuitry of thedynamic analyzer is arranged that the readout device 232 has a linearinput voltage relationship along the X axis. This relationship isprecisely set at potentiometer 273 to the decades through which the logI changes as represented by the potential signal iV= log I at output 213of the bipolar log converter 171. In this manner, the semilog readoutalong the X axis provides the magnitude of current I for a given sweepvoltage signal V,,.

In the absolute value circuit of FIG. 6, the resistors 257, 258, 266 and267, are arranged so that the amplifier 251 operates in only one DCpolarity by adjustment of the potentiometer 256. When the potentialsignal is positive +V, the amplifier 251 has unity gain and the output262 is negative in polarity and point 259 is exactly of the samemagnitude as the potential signal but negative in polarity. At thiscondition, current can flow through the diode 261 to the summing point259 at twice the magnitude of the current through resistor 263 butopposite in direction. The voltage follower amplifier 252 now produces acorresponding positive potential signal at the terminal 274 forapplication to the readout device 232. When the potential signal isnegative in polarity V, the unity gain amplifier 251 is inactive sincepoint 259 remains at zero potential. Current can flow to the summingpoint 269 only from input 254. As a result, the voltage followeramplifier 252 produces a like magnitude positive potential signal at theoutput 274 for application to the readout device 232. Either of theinput signals, irrespective of whether they are positive or negative inpolarity, produces identical positive potential signals V I at theoutput 274 of the absolute value circuit 231. The resistance network isadjusted to provide this result so that the relationship between currentI and potential signal V, are maintained through several decades ofchange in the current signal. In general, the resistors 257 and 266 haveequal values, and the resistor 267 has a value one-half the magnitude ofeither of the resistors 257 and 266. This particular arrangementprovides for precise conversion of a positive or negative potentialsignal to an identical positive potential signal of equal magnitude andin exact calibration to the required decades corresponding to thebidirectional current flows applied to the bipolar log converter 171.Thus, the dynamic-analyzer maintains exact calibration and relationshipsbetween currents of logarithmic characteristic and linear sweep voltagethroughout any desired selected operating range by adjustment of circuitvalues.

The present dynamic analyzer has many applications in the field ofelectrochemistry and electroanalytical instrumentation. The presentinvention produces by selected programming, or manual operation, bothpotentiodynamic and potentiostatic measurements in any suitable externalsystem such as an electrochemical cell or corrosion test cell. In apotentiodynamic mode.

the present dynamic analyzer causes a current flow in the cell 11between terminals C and D such that the induced potential betweenterminals A and B follow a preselected linear sweep voltage rate betweentwo voltage limits. The sweep rate and the voltage limits may beprogrammed individually by the operator. In the potentiostatic mode, thepresent dynamic analyzer causes a current flow in the external systemsuch that the induced polarization potential is held constant at a valuedetermined by the operator. Anodic and cathodic currents can becontinuously determined and recorded as a permanent readout for a logcurrent function over several decades of change. In one embodiment ofthe present invention, the log current function could be recorded over arange of five decades with a sweep voltage signal ranging eight volts.Sweep rates between l0 millivolts per hour and 100 volts per hour werereadily obtained. Zero crossing characteristics produced by the bipolarlog converter are substantially free of distortion down within a fewnanoamperes of current.

The present dynamic analyzer is applicable to the Various modificationsand alterations in the bipolar log converter are apparent to thoseskilled in the art from the foregoing description which do not departfrom the spirit of the invention. For this reason, these changes inelements and functioning are desired to be included within the scope ofthe present invention. The appended claims define the present inventionand the foregoing description is to 'be employed for setting forth thespecific embodiments as illustrative in nature.

What is claimed is:

l. A bipolar log converter comprising a. a differential input amplifierhaving one input receiving a bidirectional current signal and anotherinput connected to circuit common of a DC power supply means, and anoutput providing a voltage representative of said current signal;

b. a pair of transistors comprising PNP and NPN types with their basesat circuit common and their collectors connected to said input of saiddifferential input amplifier receiving the current signal;

. avoltage biasing network for providing a current flow from said DCpower supply means through the collector-emitter junctions of saidtransistors thereby placing said transistors into a conducting conditionfor nonlinear potential-current conversion;

d. a pair of diodes series connected between the emitters of saidtransistors and at the common junction of said diodes connected to theoutput of 60 said differential input amplifier thereby forming afeedback loop wherein said current signal is passed through one of saidcollector-emitter junctions and a diode to the output of saiddifferential input amplifier for creating a potential; and

e. means for providing from said potential a continuous potential signalrepresenting the current signal whereby said potential signal has apolarity representative of the flow direction of the current signal anda magnitude representative of the logarithm of the current signal, andsaid potential signal changing polarity upon a change in flow directionof the current signal.

2. The bipolar log converter of claim 1 wherein the voltage biasingnetwork includes a pair of shunting resistances in parallel with saidtransistors whereby the current flows through the collector-emitterjunctions and through said shunting resistances are in a ratio of atleast 1 to 1,000 so that the current flow through the collector-emitterjunction remains substantially constant.

3. The bipolar log converter of claim 2 wherein the ratio of currentflows is at least 1 to 100,000.

4. The bipolar log converter of claim 2 wherein a resistance bridge isconnected between said emitters of said transistors to produce apotential signal corresponding to said potential created at one of saidcollector-emitter junctions.

5. The bipolar log converter of claim 4 wherein a voltage followerdifferential amplifier receives said potential signal at one input andthe other input is connected through a temperature compensatingresistance to circuit common and in a feedback loop to the outputthereof whereby the voltage output signal at said output isrepresentative of the current signal and corrected for temperatureerrors in said resistance bridge.

6. The bipolar long converter of claim 1 wherein the voltage biasingnetwork includes a four-arm resistance bridge, first and secondresistances in parallel with said transistors, and third and fourthresistances in parallel with said first and second resistances, thecurrent flows through said collector-emitter junctions and through saidfirst and second resistances being in a ratio of at least 1 to 1,000whereby the current flow through said collector-emitter junctionsremains substantially constant, said third and fourth resistancesforming a resistance bridge to produce at the junction of said third andfourth resistances a potential signal corresponding to said potentialcreated at one of said collector-emitter junctions.

7. The bipolar log converter of claim 6 wherein a voltage followerdifferential amplifier receives said output voltage at one input and theother input connected through a tempeature compensating resistance tocircuit common and in a feedback loop to the output thereof whereby thepotential signal at said output is representative of the current signaland corrected for temperature errors in said resistance bridge beforeapplying said potential signal to said means for providing an outputthereof.

8. The bipolar log converter of claim 6 wherein a second pair oftransistors comprising PNP and NPN types are connected emitter-emitterwith like types mounted on a common chip with the first mentioned pairof transistors, and said second pair of transistors connected with theircollectors to the junction of said first and second resistances and withtheir bases connected in reverse phase to the junctions of said firstand third, and second and fourth resistances whereby a temperaturechange in one of the collector-emitter junctions produces anout-of-phase current through one of said first and second resistances ofsaid bridge thereby compensating for a temperature induced-currentvariation in the nonlinear potential-current conversion at thecollector-emitter junctions.

9. A bipolar log converter comprising:

a. a differential input amplifier having one input receiving abidirectional current signal and another input connected to circuitcommon of a DC power supply means, and an output providing a voltagerepresentative of said current signal;

b. a pair of transistors comprising PNP and NPN types with their basesat circuit common and their collectors connected to said input of saiddifferential input amplifier receiving the current signal;

c. a voltage biasing network connected between positive and negativeterminals of said DC power supply means and said transistors havingtheir emitters connected between said positive and negative terminalswhereby a current flow between the collector-emitter junctions placessaid transistors into a conducting condition for nonlinearpotential-current conversion; I

d. a pair of diodes series connected between said emitters of saidtransistors and the common junction between said diodes being connectedto the output of said differential input amplifier forming a feedbackloop current, and said diodes selectively gating said current signalthrough one of said collector-emitter junctions into said feedback loopfor conversion of said current signal into a potential whose polarity isdetermined by which transistor has conducted said current signal;

. a resistance bridge connected between said emitters of saidtransistors producing a potential signal corresponding in polarity tothe potential produced by one of the collector-emitter junctions; andmeans for providing a scaler indicia of said potential signal from saidresistance bridge which indicia is the logarithm of the magnitude ofsaid current signal applied to said differential input amplifier.

10. The bipolar log converter of claim 9 wherein the voltage biasingnetwork includes a four-arm resistance bridge, first and secondresistances in parallel with the emitters of said transistors, and thirdand fourth resistances in parallel with said first and secondresistances, current control means associated with said first and secondresistances providing a low impedance current path in parallel with thecollector-emitter junctions of said transistors whereby the currentflows through the collector-emitter junctions and through the lowimpedance current path of the first and second resistances is in a ratioof at least 1 to 1,000 so that the current flow through thecollector-emitter junctions remains substantially constant, and saidthird and fourth resistances form the resistance bridge to produce atthe junction of said third and fourth resistances an output voltagecorresponding to said antilog potential signal created at one of saidcollector-emitter junctions.

11. The bipolar log converter in claim 10 wherein said current controlmeans vary current flow responsively to temperature changes in saidfirst and second resistances whereby the current flow in saidcollectoremitter junctions remains substantially constant.

12. The bipolar log converter of claim 10 wherein said current controlmeans include a second pair of transistors comprising PNP and NPN typesconnected emitter-emitter with like types mounted on a common chip withthe first mentioned pair of transistors, and said second pair oftransistors connected with their collectors to the junction of saidfirst and second resistances and with their bases connected in reversephase to the junction of said first and third, and second and fourthresistances whereby the collector-emitter junctions of said second pairof transistors provide a low impedance current path and a temperaturechange in one of the collector-emitter junctions produces an outof-phasecurrent through one of said first and second resistances of said bridgethereby compensating for a temperature induced-current variation in thenonlinear potential current conversion at the collector-emitterjunctions.

13. The bipolar log converter of claim 9 wherein the ratio of currentflows is at least 1 to 100,000.

14. The bipolar log converter of claim 9 wherein a voltage followerdifferential amplifier receives at one input the potential signal fromsaid resistance bridge and the other input is connected through atemperature compensating resistance to circuit common and in a feedbackloop to the output thereof whereby the potential signal at said outputis representative of the current signal and corrected for temperatureerrors in said resistance bridge before applying said potential signalto said means for providing a readout.

15. The bipolar log converter of claim 9 wherein a feedback capacitanceshunts the output and said one input receiving said current signal ofsaid differential input amplifier whereby said capacitance compensatesfor rapid current polarity reversals as conduction terminates in one ofsaid diodes and begins in the other of said diodes.

16. The bipolar log converter of claim 15 wherein a voltage followeramplifier is connected a first input to said output of said differentialinput amplifier and a second input to circuit common and said voltagefollower amplifier has a feedback loop between said first input andoutput thereof and a series capacitance providing a current connectionto said one input receiving said current signal of said differentialinput amplifier, and said series capacitance being a small fraction ofsaid feedback capacitance of said differential input amplifier wherebythe output voltage of said differential input amplifier moves smoothlyvoltagewise at slow rates of transition in conduction of said diodes andsaid voltage follower amplifier at fast rates of transition inconduction of said diodes has current gain to act through said seriescapacitance to regulate the output voltage of said differential inputwhich moves voltagewise at a comparable rate to a slow rate oftransition in conduction of said diodes.

UNITED STATES PATENT OFFICE CERTIFIQATE 0F CORRECTION PATENT NO.3,928,774

DATED December 23, l975 iNvENToms) Homer M. Wilson It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column l, line 67, for "voltagecurrent" read ---voltage-current---;

Column 2, line 50, for "linearlogarithmic" read ---l1'near-logarithmic-g Column l2, line 29, for "7l" read ---l7l--;

line 44, after "illustrated", insert ---a preferred---; and line 54, for"puncture" read ---juncture---;

Column l3, line 60, for "eollectors" read ---c0llectors--; and

Column 14, line 49, for "at" read --as---.

Signed and Sealed this A ttesr:

RUTH C. MASON- C. MARSHALL DANN Arresting Ojj'icrr Commissionernflarenrs and Trademarks

1. A bipolar log converter comprising a. a differential input amplifier having one input receiving a bidirectional current signal and another input connected to circuit common of a DC power supply means, and an output providing a voltage representative of said current signal; b. a pair of transistors comprising PNP and NPN types with their bases at circuit common and their collectors connected to said input of said differential input amplifier receiving the current signal; c. a voltage biasing network for providing a current flow from said DC power supply means through the collector-emitter junctions of said transistors thereby placing said transistors into a conducting condition for nonlinear potential-current conversion; d. a pair of diodes series connected between the emitters of said transistors and at the common junction of said diodes connected to the output of said differential input amplifier thereby forming a feedback loop wherein said current signal is passed through one of said collector-emitter junctions and a diode to the output of said differential input amplifier for creating a potential; and e. means for providing from said potential a continuous potential signal representing the current signal whereby said potential signal has a polarity representative of the flow direction of the current signal and a magnitude representative of the logarithm of the current signal, and said potential signal changing polarity upon a change in flow direction of the current signal.
 2. The bipolar log converter of claim 1 wherein the voltage biasing nEtwork includes a pair of shunting resistances in parallel with said transistors whereby the current flows through the collector-emitter junctions and through said shunting resistances are in a ratio of at least 1 to 1,000 so that the current flow through the collector-emitter junction remains substantially constant.
 3. The bipolar log converter of claim 2 wherein the ratio of current flows is at least 1 to 100,000.
 4. The bipolar log converter of claim 2 wherein a resistance bridge is connected between said emitters of said transistors to produce a potential signal corresponding to said potential created at one of said collector-emitter junctions.
 5. The bipolar log converter of claim 4 wherein a voltage follower differential amplifier receives said potential signal at one input and the other input is connected through a temperature compensating resistance to circuit common and in a feedback loop to the output thereof whereby the voltage output signal at said output is representative of the current signal and corrected for temperature errors in said resistance bridge.
 6. The bipolar long converter of claim 1 wherein the voltage biasing network includes a four-arm resistance bridge, first and second resistances in parallel with said transistors, and third and fourth resistances in parallel with said first and second resistances, the current flows through said collector-emitter junctions and through said first and second resistances being in a ratio of at least 1 to 1,000 whereby the current flow through said collector-emitter junctions remains substantially constant, said third and fourth resistances forming a resistance bridge to produce at the junction of said third and fourth resistances a potential signal corresponding to said potential created at one of said collector-emitter junctions.
 7. The bipolar log converter of claim 6 wherein a voltage follower differential amplifier receives said output voltage at one input and the other input connected through a tempeature compensating resistance to circuit common and in a feedback loop to the output thereof whereby the potential signal at said output is representative of the current signal and corrected for temperature errors in said resistance bridge before applying said potential signal to said means for providing an output thereof.
 8. The bipolar log converter of claim 6 wherein a second pair of transistors comprising PNP and NPN types are connected emitter-emitter with like types mounted on a common chip with the first mentioned pair of transistors, and said second pair of transistors connected with their collectors to the junction of said first and second resistances and with their bases connected in reverse phase to the junctions of said first and third, and second and fourth resistances whereby a temperature change in one of the collector-emitter junctions produces an out-of-phase current through one of said first and second resistances of said bridge thereby compensating for a temperature induced-current variation in the nonlinear potential-current conversion at the collector-emitter junctions.
 9. A bipolar log converter comprising: a. a differential input amplifier having one input receiving a bidirectional current signal and another input connected to circuit common of a DC power supply means, and an output providing a voltage representative of said current signal; b. a pair of transistors comprising PNP and NPN types with their bases at circuit common and their collectors connected to said input of said differential input amplifier receiving the current signal; c. a voltage biasing network connected between positive and negative terminals of said DC power supply means and said transistors having their emitters connected between said positive and negative terminals whereby a current flow between the collector-emitter junctions places said transistors into a conducting condition for nonlinear potential-current conversion; d. a pair of diodEs series connected between said emitters of said transistors and the common junction between said diodes being connected to the output of said differential input amplifier forming a feedback loop current, and said diodes selectively gating said current signal through one of said collector-emitter junctions into said feedback loop for conversion of said current signal into a potential whose polarity is determined by which transistor has conducted said current signal; e. a resistance bridge connected between said emitters of said transistors producing a potential signal corresponding in polarity to the potential produced by one of the collector-emitter junctions; and f. means for providing a scaler indicia of said potential signal from said resistance bridge which indicia is the logarithm of the magnitude of said current signal applied to said differential input amplifier.
 10. The bipolar log converter of claim 9 wherein the voltage biasing network includes a four-arm resistance bridge, first and second resistances in parallel with the emitters of said transistors, and third and fourth resistances in parallel with said first and second resistances, current control means associated with said first and second resistances providing a low impedance current path in parallel with the collector-emitter junctions of said transistors whereby the current flows through the collector-emitter junctions and through the low impedance current path of the first and second resistances is in a ratio of at least 1 to 1,000 so that the current flow through the collector-emitter junctions remains substantially constant, and said third and fourth resistances form the resistance bridge to produce at the junction of said third and fourth resistances an output voltage corresponding to said antilog potential signal created at one of said collector-emitter junctions.
 11. The bipolar log converter in claim 10 wherein said current control means vary current flow responsively to temperature changes in said first and second resistances whereby the current flow in said collector-emitter junctions remains substantially constant.
 12. The bipolar log converter of claim 10 wherein said current control means include a second pair of transistors comprising PNP and NPN types connected emitter-emitter with like types mounted on a common chip with the first mentioned pair of transistors, and said second pair of transistors connected with their collectors to the junction of said first and second resistances and with their bases connected in reverse phase to the junction of said first and third, and second and fourth resistances whereby the collector-emitter junctions of said second pair of transistors provide a low impedance current path and a temperature change in one of the collector-emitter junctions produces an out-of-phase current through one of said first and second resistances of said bridge thereby compensating for a temperature induced-current variation in the nonlinear potential current conversion at the collector-emitter junctions.
 13. The bipolar log converter of claim 9 wherein the ratio of current flows is at least 1 to 100,000.
 14. The bipolar log converter of claim 9 wherein a voltage follower differential amplifier receives at one input the potential signal from said resistance bridge and the other input is connected through a temperature compensating resistance to circuit common and in a feedback loop to the output thereof whereby the potential signal at said output is representative of the current signal and corrected for temperature errors in said resistance bridge before applying said potential signal to said means for providing a readout.
 15. The bipolar log converter of claim 9 wherein a feedback capacitance shunts the output and said one input receiving said current signal of said differential input amplifier whereby said capacitance compensates for rapid current polarity reversals as conduction terminates in one of said diodes and begins in The other of said diodes.
 16. The bipolar log converter of claim 15 wherein a voltage follower amplifier is connected a first input to said output of said differential input amplifier and a second input to circuit common and said voltage follower amplifier has a feedback loop between said first input and output thereof and a series capacitance providing a current connection to said one input receiving said current signal of said differential input amplifier, and said series capacitance being a small fraction of said feedback capacitance of said differential input amplifier whereby the output voltage of said differential input amplifier moves smoothly voltagewise at slow rates of transition in conduction of said diodes and said voltage follower amplifier at fast rates of transition in conduction of said diodes has current gain to act through said series capacitance to regulate the output voltage of said differential input which moves voltagewise at a comparable rate to a slow rate of transition in conduction of said diodes. 